The present invention relates to a method for regulating the heights of the electrodes in a three-phase melting furnace having freely burning electric arcs in dependence on the arc voltages derived from the phase voltages and from drops in active and reactive potential across the high current lines, with each electrode having an associated arc voltage regulator which receives the arc voltage as the actual value and a desired arc voltage value as a reference value.
Conventional electrode regulating systems for threephase electric arc furnaces primarily employ the so-called impedance regulation method. That is, for each electrode, the quotient is formed of the voltage measured between the high current path and the neutral or star point of the bath divided by the current of the respective high current path. This measured value is then the actual value used for impedance regulation. Aside from the fact that this manner of regulating tends to correctly influence the level or height of the electrode only if the deviations from normal operating states are not too great, it has a number of additional drawbacks.
Firstly, the changes taking place in a high current path, for example, due to changes in electrode position, have an effect on the measured impedance values of the other two electrodes and, in spite of their being in the correct position with respect to the desired final state, cause them to perform a reaction movement which is reversed during the course of the further regulating process. The three control circuits are not decoupled.
Moreover, the measured value (impedance) is not a reliable measure for the arc lengths. That is, in spite of the same desired impedance values, considerable differences may develop between the lengths of the three arcs.
Finally, in extreme situations, e.g. if one electrode comes into contact with the bath (scrap), undesirable electrode movement may take place with resulting damage, for example, if an electrode that is in contact with the scrap is further reduced in height, i.e., lowered, due to such error reactions, and thus electrode breakage occurs.
These disadvantageous phenomena can be avoided if a measuring value is employed which is the best possible representation of the desired setting value, namely the length of the arc, which is adjustable by changing the height of the electrode above the bath.
Federal Republic of Germany Offenlegungsschrift DE-OS 2,440,960 indicates to the person skilled in the art, in FIG. 5 and the associated text, that in certain regions of the arc current one must assume that there is only a slight dependency of the arc voltage upon the current. This view has been reinforced in the meantime by a dissertation by Werner Hoke, entitled "Der Spannungsbedarf des stationaren Hochstromlichtbogens in einem Lichtbogenofen fur die Stahlherstellung" (Voltage Requirement for the Stationary High Current Arc in an Electric Arc Furnace for the Production of Steel) dated July 29, 1980 and by extensive theoretical examinations, the results of which are discussed (see particularly pages 77-83) and are shown in FIG. 18 of that dissertation. In a range between about 15 to 80 kA, the substantial constancy of the arc voltage is confirmed. It is therefore appropriate to assume that in medium and large size electric arc furnaces operating with current intensities up to about 70 kA, the voltage drop of the arc does in effect take on an almost constant value shortly after the current passes through zero. This is confirmed by numerous oscillograph arc voltage curves published in the literature. To decouple, on the one hand, the three high current circuits with respect to electrode regulation and, on the other hand, to avoid imprecise or erroneous reactions, it is therefore appropriate to use the arc voltage as the direct variable for electrode regulation, particularly since changes in the distance between the electrode and the bath are clearly reflected in the sign of the change of the average arc voltage.
The arc voltage can be determined by measuring, for example, according to the teaching of Federal Republic of Germany Auslegeschrift No. DE-AS 2,405,252 in conjunction with the teaching of Federal Republic of Germany Offenlegungsschrift No. DE-OS 2,437,557. With this measuring method it would be possible to use the arc resistance (quotient of arc voltage over arc current) as the regulating variable instead of the above-described impedance. This would initially have the advantage that when all electrodes are lowered in the currentless state, the measured arc voltage and the measured arc current are zero for the electrode which is the first to come in contact with the scrap. This results in an undetermined quotient and thus, correctly, no actuation of the electrode regulation, i.e., the electrodes remain stationary.
In the case of pure voltage regulation, however, this operating state would lead to the result that the electrode, which first comes in contact with the bath, would dance up and down. This results from the fact that in the raised state, the effective mains voltage to neutral is greater than the desired value of the arc voltage, while upon contact with the bath the situation is reversed.